Since then, the Syrian government appears to have taken the upper hand in the conflict. That was most evident when rebels gave up their stronghold of Homs in a ceasefire deal with the government.

In a short televised statement, Brahimi said he was sure that there would be an end to the conflict.

"The question is only this: everybody who has responsibility and an influence in the situation has to remember that the question is how many more dead? How much more destruction is there going to be before Syria becomes again the Syria we have known – the new Syria that will be different from the Syria of the past, but it will be the Syria we have loved and admired for many, many years," Brahimi said.

United Nations Secretary-General Ban Ki-moon did not hide his disappointment, saying he had not decided on a replacement and conceding that the two high-profile envoys had "not been able to make any progress."

Ban blamed strong divisions both inside Syria and at the U.N., where Western powers have been unable to sway Russia into signing on to any sanctions or actions against Assad.

"I greatly appreciate Mr. Brahimi's diplomacy in organizing the Geneva Conference on Syria and for facilitating the intra-Syria talks earlier this year," Ban said. "I regret that the parties, and especially the Government, have proven so reluctant to take advantage of that opportunity to end the country's profound misery. I renew my appeal to them to show the wisdom and sense of responsibility that could allow a way out of this nightmare."